Preguntas frecuentes

The battery of hybrid and pure electric vehicles consists of many electrochemical cells electrically connected in a series string. The characteristics of each cell, like capacity, internal resistance, self-discharge or end-of-charge-voltage are never precisely equal.

Even new cells have differences. Charging them in a series connection will always result in one to be the first and one to be the last fully charged. [21] Either the first cell is being overcharged or the last lacks to be fully charged or both. This effect dramatically rises with the number of discharge-charge cycles of the battery leading to reduced performance and shorter battery life. This effect will increase if the cells are on different temperatures. Battery equalization can reduce or eliminate these effects and reviewing them is essential for prolonging the life of the batteries and keeping the running cost low. Complex equalization systems on the other hand can increase the purchase cost of vehicles significantly.

Figure 2-2 shows a string of three old Hawker Genesis blocks in a series connection while charging, discharging and idle periods without an equalizer. The figure shows the three block voltages and the charging current. It can be seen that block 1 is not fully charged while block 3 is overcharged (valve openings result in voltage bursts).

Figure 2-2: Charging of Three Old Battery Blocks in Series without Charge Equalization

There are different methods and topologies for equalizing the cells in a series connection. They are discussed in the following subsections.

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2.5.1 Equalization Methods

The string equalization method is the natural method for all good-natured chemistries. It is based on the rise of internal energy dissipation once a cell is fully charged. It works by a long and careful overcharge until all cells are fully charged. It cannot be applied to Li-Ion batteries.

The idea of dissipative equalization or current shunting is to draw energy from the fullest cells or blocks and dissipate it in a resistor or transistor. Figure 2-3 shows this principle. The control circuit controls this device to keep all cells in the battery string equalized.

Figure 2-3: Working Principle of a Dissipative Equalizer

The main advantages of this method are simplicity and low cost, but the equalizing current is very limited.

The method of switched reactors is based on transferring energy from the cells with higher voltage to its neighbor with lower charge. This method works bi-directionally, usually comparing two neighboring blocks. A Daisy Chain assures all blocks are equalized. High equalizing currents are achievable without large heat sinks. Figure 2-4 shows one circuit of a switched reactor in principle. It equalizes two batteries.

N-1 circuits in a Daisy Chain are needed to equalize N batteries. The transistor next to the block or cell with higher charge is controlled with a PWM. When switched on (phase 1) it draws current from this block through a reactor, which “stores this current”. When switched off (phase 2), the neighbored block is charged with this small amount of stored energy.

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Figure 2-4: Working Principle of Equalizing with Switched Reactors

The main drawback of this method is the higher complexity and the fact that the cell voltages are compared with their neighbors and not with a reference.

Figure 2-5 shows the method of flying capacitors in principle: The switches switch back and fro with a certain frequency. The capacitor “between” two blocks reaches the average voltage of these blocks. It discharges the block with higher voltage in first step and charges the block with lower voltage in the second step.

Block 1

Block 2

Block n

C1,2

C2,3

Cn-1,n

Figure 2-5: Working Principle of Equalizing with Flying Capacitors

This technology is comparatively complex and it will not achieve high equalizing currents. The control algorithm is very simple, but cell voltages can never be fully equalized due to the exponential charge/discharge characteristic of capacitors. The circuit can be simplified for smaller cell numbers by using only one capacitor that is switched among all cells using a multiplexer. This would be a very low-cost solution for up to 16 cells in series.

31 Another method is to charge each cell or block separately with its own charger or to distribute additional small amounts of energy to some cells/blocks when necessary.

Providing a single charger for each cell or block of course means very high complexity, space, wiring (high current cables) and cost. Distributing only small amounts of energy to the low cells or blocks can be combined with cell or block voltage measurement, using the sensing cables.

Figure 2-6: Isolated Distributing Equalizer Circuit [22]

Figure 2-6 shows a circuit using a bus system. High frequency transformers are used for communication and for distributing small amounts of energy for equalizing. These methods are very complex but provide the fastest equalization and charging.

2.5.2 Topology of Equalizer

There are different possible topologies for battery equalizing systems:

• Centralized Equalizer

• Partly centralized Equalizer

• Modular Equalizer

• Master-Slave Architecture

All might provide an interface to the charger, driver or other components or might be without any interface. The partly centralized equalizer and the modular equalizer can or cannot be linked for communication.

32 The centralized solution does not need a link like a bus-system for communication between modules. It has to cope with high voltage drops in one housing, if applied to a battery with many blocks and it is not scalable to the number of blocks in different batteries.

Figure 2-7: Central Equalizer with Interface

The centralized equalizer as shown in Figure 2-7 is a simple and reliable solution for batteries with a few cells and comparatively low voltages up to 70V. High production volumes for a certain number of blocks, like the new 42V system in cars, will favor this solution due to its lower price.

Having a battery with a higher voltage and more blocks requires the equalizer to be split into several parts, as shown in Figure 2-8. This topology requires an isolated interface or bus system between the modules, if communication or external interface is needed. The BEMU from SKI [23] uses this topology.

Figure 2-8: Partly Centralized Equalizer with Interface

In some cases like for the PowerCheq from PowerDesigners [19] without any communication or interface it is sensible to have a topology with one standalone module per block or between two blocks. This modular approach as shown in Figure 2-9 makes the system fully scalable with the number of blocks in any battery, thus reducing cost because of higher possible production volumes. No long wires are required and the temperature gradients on long battery strings can easily be taken into account by attaching the equalizer to each cell. If any interfacing or communication is

33 required on the other hand, this topology will suffer from higher complexity and thus higher cost.

Figure 2-9: Modular Equalizer without Interface

Master-Slave architectures combine some advantages of the modular and the centralized topology. The central solution becomes scalable to the number of blocks or cells and the modular solution receives capabilities like interface and communication without high cost.

The suitable topology of the equalizer does not influence the quality or the results of equalization. It needs to be chosen regarding cost, number of blocks to be equalized, battery voltage, production volume, circuit design, interfacing, equalizing method and battery type. The general requirements for battery cell equalization depend on several factors:

• The battery technology (chemistry, size, type)

• The application

• Number of cells in series connection

• Charging methods

2.5.3 Equalizer for Li-Ion Batteries

Li-Ion batteries require comparatively small equalizing currents, because the equalization needs to offset the differences in self-discharge rates only. The cell voltages are monitored anyway in order to prevent damage to the batteries.

The central solution with flying capacitor method is the simplest and most promising solution for battery strings up to 8 or 16 cells. One capacitor can be connected to the output of an off-the-shelve multiplexer with 8 inputs. This multiplexer is required for connecting the chosen cell to the cell monitoring system. The internal resistance of available multiplexer is above 50 ohms and limits the equalizing currents. Cells with very high capacity on the other hand might require higher equalizing currents. It needs to be found, what cell size requires what equalizing current over the lifetime.

The central solution with current shunting method is preferred for small cell number of up to 16 cells. This method will also work for larger capacities, because currents can easily be in the order of 120 mA and this is much more than the self-discharge rate for Li-Ion batteries. A resistor in parallel to the cell with the highest voltage can

Block 1

34 be switched on till this cell-voltage equals the lowest cell-voltage in the string. The current drawn is small enough not to influence the cell-voltage instantaneously. The voltage drops slowly through reducing the state of charge.

Current shunting is also suitable for higher cell numbers, but master-slave architecture is required. Available cost-effective multiplexer can handle up to about 70V or 16 Li-Ion cells, communication is required for monitoring cell-voltages and temperatures and interfacing to a battery management system. It is sensible to keep the number of cells in the battery as low as possible in order to minimize the complexity and cost for the equalizing system.

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